External beam radiation therapy systems provide beams of high-energy directed into a patient to treat tumors or the like. The size, location, angle and intensity of the beams are determined by a treatment plan based upon the precise measurement of the dose to be delivered to the patient to provide for precise control of the dose to the patient.
Quantitative accuracy is ensured by periodic calibration of the machine using radiation detectors and phantoms to determine the relationship between the control settings of the machine and actual dose. One type of radiation detector is an ionization chamber in which electrodes are placed on opposite sides of a volume of gas. The gas is ionized by the radiation passing through the chamber volume and the ions are collected on one collector electrode under the influence of a voltage applied across the electrodes.
Ideally the ionization chamber volume is small compared to the beam size to limit “partial volume” errors that affect the ionization chamber measurement when the volume is not fully irradiated by the measured radiation beam.
Certain radiation therapy systems, for example, the Gamma Knife® or a linear accelerator configured for stereotactic radiotherapy or radiosurgery, provide extremely small radiation beams, for example, as small as 4 mm. It is difficult to construct ionization detectors that are small enough to avoid partial volume effects while providing desired sensitivity.